Carbon nanotube composite membrane

ABSTRACT

A composite membrane for separations includes a fabric with a non-woven array of intermingled carbon nanotubes, and a dopant incorporated with the fabric to form a non-porous, permeable composite. The composite membrane may be used to separate a target gas from a liquid by mounting the composite membrane in a housing chamber, and conditioning a permeate side of the chamber to establish a driving force for the target gas across the non-porous, permeable composite membrane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/911,433, filed on Jun. 6, 2013 and entitled “Carbon NanotubeComposite Membrane”, the contents of which being incorporated herein intheir entirety.

FIELD OF THE INVENTION

The present invention relates to gas separation and ion transportmembranes generally, and more particularly to a composite membranestructure having a backbone of an array of nanotubes.

BACKGROUND OF THE INVENTION

A long list of polymers have been studied in the past for their utilityin the formation of membranes for a variety of purposes. Of importanceto the present invention is the use of such polymer membranes forseparations and selective transport to modify a feed material and/or torecover one or more target species from the feed material. A host ofpolymers have been determined to be useful for such applications, witheach material exhibiting its own benefits and drawbacks for particularmembrane separation and transport applications.

An application of particular interest to the Applicant is the degassingof liquids through contact with a gas-permeable, liquid-impermeablemembrane. Such liquid-gas contactors typically rely upon Henry's Law ofpartial pressures and Ficke's law of diffusion to drive gas transportthrough the membrane, while small pore size, or the absence ofthrough-pores in a “nonporous” media, restricts or prevents liquidtransport through the membrane. The development of fluoropolymers hasgreatly aided the membrane liquid degassing field by providing membranepolymers that are generally inert, and can be formed into agas-permeable, liquid-impermeable membrane structure. A particularfluoropolymer of note is a class of amorphous perfluoropolymers, such asthose available from Du Pont under the trade name “Teflon®”, as well asother amorphous fluoropolymers available from Asahi Glass Corporationand Solvay Solexis. Such materials are oftentimes employed in gasseparation membranes for their inertness and high permeabilitycharacteristics. Membranes are typically selected for a combination oftheir compatibility with the contacting materials, their permeability tothe targeted transport species, and their selectivity of one moleculeover another. It has been shown that, while membrane selectivity may beconstant as a function of the membrane thickness, the throughput(permeance) changes inversely to the thickness of the membrane. As aresult, a thinner membrane is typically desired, but is limited by thedecreased strength and durability as membrane thickness is reduced. Itis therefore an ongoing challenge to obtain selective membranes thathave the highest possible permeance without being unduly fragile. Suchmembranes should also be resistant to fouling, degradation, or otherperformance deterioration.

Membrane engineers have attempted to employ fragile membranes withdesired performance properties by supporting the selective membraneswith a support structure. A variety of reinforcing support structureshave been previously implemented, but are typically difficult to handle,expensive, and/or degrade the performance of the primary selectivemembrane. Suitable structural reinforcements to thin film membranes thatavoid these drawbacks have yet to be defined.

Reinforcement materials for thin film membranes have typically been inthe form of lattice structures, support films, and particulate dopants.One material that has been extensively studied for its strengtheningproperties, though not in thin film separation membranes, is carbonnanotubes, which are recognized as a high-strength material, derivingtheir strength from its native sp² bond structure. The electron cloudassociated with the sp² bonding structure functions as an interactionbetween proximate carbon nanotubes, such that nanotubes may be formedinto coherent sheets, tapes, ribbons, ropes, and other macrofabrics,with a tensile strength that is sufficient to facilitate handling.

Internanotube forces have been noted so long as the nanotubes are wellassociated wherein the surfaces of proximate nanotubes can interact. Theparticular method of forming nanotubes into such sheets, tapes, and thelike, however, can greatly affect the strength of the so-formedmacro-scale nanotube structures. Carbon nanotube arrays in a sheet formare commonly known as “buckypaper”, which owes its name tobuckminsterfullerene, the 60 carbon fullerene (an allotrope of carbonwith similar bonding that is sometimes referred to as “Bucky ball” inhonor of Buckminster Fuller). Generally, the bonding interactions amongthe nanotubes are insufficient to form a buckypaper that has commercialuse on its own. However, carbon nanotubes have been described as adopant to various materials, including polymers, by mixing carbonnanotube powder into the polymer. Typically, researchers seek improvedstrength and/or electrical conductivity when doping polymers with carbonnanotubes.

Strength reinforcement materials, including glass fibers, carbon fibers,metal fibers, carbon nanotubes, and the like, when conventionally addedas a reinforcement material, are dependent upon surface energycompatibility between the reinforcement material and the matrix for thedegree of strength enhancement. Matching of the respective surfaceenergies permits van der Walls interactions to assist in the loadtransfer between the reinforcement material and the matrix. In somecases, surface energy matching is not possible without chemicalmodification of the reinforcing material, which chemical modificationcan be expensive or even impossible.

Recently, a reverse approach has been attempted, wherein a buckypaper isinfused with a polymer to maintain the native strength of the carbonnanotube sheet derived from the van der Walls interactions among thenanotubes. An example of such an approach is described in U.S. Pat. No.7,993,620, herein incorporated by reference. The non-woven carbonnanotube fabric described in U.S. Pat. No. 7,993,620 may be incorporatedinto composite structures by impregnating the non-woven fabric with amatrix precursor, and allowing the matrix to polymerize or thermallycure. Such composites have been described for use in impact-resistantapplications, such as sporting goods protection devices, includinghelmets. Other carbon nanotube fabric composites are described in U.S.Patent Application Publication No. 2010/0324565, also incorporatedherein by reference.

The example carbon nanotube fabric structure described above for theformation of composites is described in detail in U.S. PatentApplication Publication Nos. 2009/0215344, and 2011/0316183, while anapparatus useful to synthesize nanotubes of such carbon nanotube fabricsis described in U.S. Patent Application Publication No. 2009/0117025,each of which are incorporated herein by reference.

Though composites of carbon nanotube fabrics and infused polymers havebeen demonstrated, the Applicant is unaware of such composites preparedas a thin film membrane for, as an example, separations. One explanationfor the lack of work in this area may be due to the expectation thatcarbon nanotube fabrics would act similarly to other reinforcementstructures that interfere with the overall permeability of the compositestructure. It is well know that solid portions of conventional thin filmsupport structures often reduce permeance performance as compared to theneat thin film separation membrane.

It is therefore an object of the present invention to provide acomposite structure that exhibits desired tensile strength with asubstantially reduced effective polymer film thickness.

It is another object of the present invention to provide a compositemembrane incorporating a nanotube reinforcement structure that does notsignificantly degrade permeation performance of the separation polymermatrix.

SUMMARY OF THE INVENTION

By means of the present invention, a thin membrane for separations maybe prepared with significantly less polymer material while maintaining,or even enhancing strength properties of the membrane. By reducing thepolymer usage, separation membrane cost may correspondingly besignificantly reduced. The reinforcement structure of this compositemembrane is a non-woven carbon nanotube fabric that is single ormultiple layers, while the carbon nanotubes are intermingled.

In one embodiment, a method for preparing a composite gas separationmembrane for separating a gas-liquid mixture includes providing a fabrichaving a non-woven array of intermingled carbon nanotubes, wherein thenon-woven array defines interstices between the intermingled carbonnanotubes. The method further includes providing a dopant and at leastpartially immersing the fabric in the dopant. The dopant is sonicatedwith an ultrasonic transducer such that the dopant penetrates the fabricinterstices to an extent sufficient to establish a non-porous butpermeable composite structure with the fabric.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a testing apparatus for testingpermeability, permeance, and gas selectivity of composite films of thepresent invention;

FIG. 2 is a cross-sectional scanning electron microscopy (SEM) image ofa composite film of the present invention;

FIG. 3 is a cross-sectional SEM image of a composite film of the presentinvention;

FIG. 4 is a schematic illustration of a membrane separation system ofthe present invention;

FIG. 5 is a schematic illustration of a membrane ion exchange system ofthe present invention;

FIG. 6 is a photograph of a cross-section of a sample composite membranetaken under scanning electron microscopy;

FIG. 7 is a spectral analysis chart illustrating the presence of carbon,fluorine, and oxygen at respective spectral analysis locationsidentified in the photograph of FIG. 6;

FIG. 8 is a spectral analysis chart illustrating the presence of carbon,fluorine, and oxygen at respective spectral analysis locationsidentified in the photograph of FIG. 6; and

FIG. 9 is a tensile strength chart illustrating Young's Modulus valuesfor various sample composite membranes, in comparison to a nonwovencarbon nanotube fabric labeled “Pure”.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features, and advances represented by the present inventionwill now be presented in terms of detailed embodiments. Otherembodiments and aspects of the invention are recognized as being withinthe grasp of those having ordinary skill in the art.

For the purposes hereof, the term “nanotubes” is intended to mean singlewall or multiple wall tubular structures having a diameter in the rangeof about 1-100 nanometers (nm), and a length in the range of about0.001-10 millimeters (mm). The nanotubes described in the experimentalsection hereof are multiple wall carbon nanotubes typically having adiameter of between about 1-25 nanometers (nm), and a length of betweenabout 0.1-5 millimeters (mm). Both single wall and multiple wall carbonnanotubes, however, are well understood in the art and are contemplatedas being useful in the present invention.

For the purposes hereof, the term “membrane” is intended to mean abarrier structure that is capable of permitting selective transportthereacross. A “composite membrane” is a membrane structure having twoor more bodies coordinating with one another in a single membranestructure. The bodies may be comprised of the same or differentmaterials, and are exemplified in this description as a polymer matrixbody incorporated with a carbon nanotube fabric.

For the purposes hereof, the term “non-porous” is intended to mean thatthe subject composite film is substantially free from open pathwaysextending continuously from one side of the composite film to the other.A “pore” is defined as an opening by which matter passes through a wallor membrane. Thus, the term “non-porous” is intended to mean the absenceor substantial absence of pores. With regard to carbon nanotubes, a“pore” could be defined as a lumen of a carbon nanotube that extendsthrough the thickness of the composite membrane to provide a pathwaythrough the composite membrane. The non-porous composite membranes ofthe present invention are free from such lumenal pathways through thecomposite membrane.

For the purposes hereof, the term “permeable” is intended to meantransport through the composite membrane that relies primarily upon asolution-diffusion mechanism, and not as a result of porosity in thestructure permitting material transport through the composite membranewithout a solution-diffusion mechanism.

As described above, carbon nanoparticles structurally similar tonanotubes have been employed in numerous applications, primarily as amechanical strengthening agent. In some of such applications, however,the addition of carbon nanoparticles in the form of graphene hasresulted in a significant reduction in polymer film permeability.Indeed, nanoparticles have been demonstrated and intentionally used tolimit permeability in a substrate, such as in U.S. Pat. No. 7,745,528and U.S. Patent Application Publication No. 2007/0137477. Consequently,the Applicant's finding that a carbon nanotube support structure to apolymer film does not significantly inhibit permeability performance isa surprising result of the present invention.

A “buckypaper” found by the Applicant to be useful in the compositemembranes of the present invention are available from NanocompTechnologies, Inc. of Merrimack, N.H. The Nanocomp buckypapers aredescribed in, for example, U.S. Patent Application Publication Nos.2011/0316183 and 2009/0215344, as well as U.S. Pat. No. 7,993,620 as anon-woven fabric of intermingled nanotubes generated through chemicalvapor deposition (CVD) or other gas phase pyrolysis procedure, thecontents of which patents and patent applications being incorporatedherein by reference. The intermingled nanotubes are randomly dispersedin a random orientation in the non-woven fabric, with the fabric beingcomprised of multiple layers of randomly-oriented non-woven nanotubesthat, taken together, form the carbon nanotube fabric. The nanotubesmaking up the tested non-woven fabric included multiple wall nanotubeshaving an outside diameter of approximately 10-15 nm, and a length ofbetween about 0.1-5 mm randomly arranged within layers of a multiplelayer non-woven fabric weighing between 1-20 g/m². The example nanotubefabrics have between 5-95% open structure, and more preferably between30-70% open structure, with a thickness of between 1-50 micrometers.

For the purposes hereof, the term “nonwoven fabric” is intended to meana sheet, web or bat of directionally or randomly oriented fibers, bondedthrough various means, including friction and/or cohesion and/oradhesion. It is believed that the carbon nanotube fabrics describedabove are bonded through π-π electron cloud interaction among the carbonnanotubes. It is contemplated that the fabric may include partially orcompletely directionally oriented fibers, or may instead be randomlyoriented nanotubes.

A wide host of dopants may be incorporated with the nanotube fabric toform the composite membranes of the present invention. For the purposeshereof, the term “dopant” is intended to mean a material that is capableof penetrating into interstices of the non-woven fabric, and which iscapable of forming a network that is sufficient to establish anon-porous but permeable barrier. Example dopants includemonomers/polymers in neat form or in solution. The polymers described inthe following examples were selected for their inertness and relativelyhigh gas permeabilities. It is to be understood, however, that othermaterials, including other polymers, polymer blends, and non-polymersmay be utilized in the formation of the composite membranes of thepresent invention. The selected dopant material is preferably capable ofdepositing within the open interstitial space of the nanotube fabric toan extent to form a nonporous but permeable composite structure having atarget gas permeance of at least 0.1 gas permeation units (GPU). In thismanner, the composite membrane may be useful in a variety of separationsapplications, including gas-gas, gas-liquid, and liquid-liquidseparations.

The term “incorporated with” is intended to describe the relationshipbetween the dopant and the nonwoven fabric, wherein the dopant is notmerely deposited upon or adjacent to the nonwoven fabric surface, butadditionally or instead penetrated into interstices of the nonwovenfabric to the extent that, upon any necessary polymerization or cure ofthe dopant, the dopant and the nanotubes interact as a compositemembrane exhibiting significantly greater absolute strength (Young'smodulus) than the respective absolute strengths (Young's Modulus) ofeither of the polymerized/cured dopant or the nonwoven fabric alone.Such synergistic strengthening of the composite membranes of the presentinvention is a surprising and important aspect, and defines the extentof penetration of the dopant into interstices of the nonwoven fabricnecessary in order to be “incorporated with” the nanotube fabric for thepurposes of the present invention.

Penetration may be defined as the distance that the dopant penetratesinto the nonwoven fabric, and can be inferred in the following examplesby measuring the fluorine concentration through the thickness of thedried composite membrane in the case of a fluoropolymer dopant. It hasbeen determined that, in order to form the composite membranes of thepresent invention, the dopant must penetrate into the surface of thenanotube fabric. Penetration of the dopant into the nonwoven fabricdepends upon the viscosity of the dopant, including as solvated insolution, and the surface energy difference between the dopant/dopantsolution and the nonwoven fabric. Penetration is also dependent upon themolecular size of the dopant.

Dopant penetration as measured by energy dispersive x-ray analysisthrough the thickness of the membrane monitoring fluorine concentrationshowed a strength to depth of penetration relationship consistent withthe relative depth of dopant penetration. Dopant impregnations resultingin a substantially uniform dopant dispersion throughout the fabricexhibit the highest absolute strengths. However, it has been found thatsufficiently strong, permeable, but nonporous composite membranes may beformed when only one side of the nonwoven fabric is exposed to thedopant/dopant solution. In the case of a dip tank, one side of thefabric may be protected from exposure to the dopant solution by use of asolid, impermeable barrier to which the dopant solution will notpermanently adhere. The frame-attached nonwoven fabric is then exposedto the dopant solution for a period of time necessary to providesufficient penetration into the nonwoven fabric. One benefit of suchone-side impregnated fabrics, wherein the non-impregnated side of thefabric remains free of dopant, is that gasses permeating from theimpregnated side of the membrane to the non-impregnated side may bevented to atmosphere, removed, by vacuum, or purged from thenon-impregnated side of the membrane using gas pressure.

Typical composite membranes of the present invention permit at least 90%of mass transport only through a solution-diffusion mechanism, and morepreferably at least 95%, and still more preferably at least 99% of themass transport through the composite membrane only by asolution-diffusion mechanism. Such a composite membrane preferablytransports sufficient mass through the membrane per unit area as isrequired for a given application.

The permeability, permeance, and gas selectivity measurements of targetgasses, such as nitrogen, oxygen, and carbon dioxide, may be obtainedusing a constant volume/variable pressure apparatus. An example testingarrangement is illustrated in FIG. 1, wherein a testing apparatus 10includes a cell 12 having a chamber 14 that is divided into an uppercompartment 16 and a lower compartment 18 by the membrane 20 beingtested. The tested membranes were each 0.95 cm² held in place by a clamp22. Upper compartment 16 had a volume of greater than 1000 cm³, whilelower compartment 18 had a volume of 25 cm³. To test membrane 20, bothcompartments 16, 18 are first purged with an identical gas for about 30min., followed by an additional 100 kPa of the gas added to uppercompartment 16. The pressure of both compartments 16, 18 is measuredwith two Omegadyne model PX 209-015G5V pressure transducers as afunction of time. The pressure of upper compartment 16 does not changeduring the test. When the rate of pressure increase on the lowercompartment 18 reaches its pseudo-steady-state, the permeability wascalculated as follows:

$P = {\frac{(22414)(l)(V)}{(A)( {\delta\; P_{0}} )({RT})} \times \frac{\mathbb{d}p}{\mathbb{d}t}}$

Where δP₀=initial pressure difference between upper compartment 16 andlower compartment 18 (cm Hg)

-   -   V=volume of lower compartment 18 (cm³)    -   l=thickness of membrane 20 (cm)    -   A=area of membrane 20 (cm²)    -   T=absolute temperature (K)    -   R=universal gas constant

$\frac{\mathbb{d}p}{\mathbb{d}t} =$

the rate of pressure increase on lower compartment 18 atpseudo-steady-state (cm Hg/sec.)

Tensile strength testing was performed using an Instron model #3345equipped with a 1000 Newton or 5000 Newton load cell and a computersystem operating Instron series IX/S software. Corrections for Poissonsratio were not made.

TABLE 1 Specimen Specimen Tensile Measurement Thickness, width lengthPull Rate Strength Case Manufacturer Type Source (microns) (mm) (mm)(mm/min) (GPa) 1 Nanocomp Generic Nanocomp Generic Unknown Unknown 1.270.329 Thickness (mean value) 2 Nanocomp 20 Instron 72 25.4 25.4 1.270.92 gram/m² 3 Nanocomp 10 Instron 25 25.4 25.4 1.27 0.229 gram/m² 4Nanocomp 5 Instron 9 25.4 25.4 1.27 0.307 gram/m²

Tensile strengths approaching those reported by the manufacturer wereobserved as the thickness of the nanotube fabric decreased. This may bedue to the many layers of material used to make the higher weightfabrics and the apparent reduced density.

To determine the improvement in tensile strength achieved byincorporating polymers with the nanotube fabrics, the tensile strengthof the individual polymers were tested and compared to literature valueswhere available. Extruded polymer film was obtained from BioGeneral,Inc. of San Diego, Calif.

TABLE 2 Specimen Specimen Tensile Measurement Thickness width lengthPull Rate Strength Manufacturer Type Source (microns) (mm) (mm) (mm/min)(GPa) DuPont Teflon Cast film Ca. 100 Hysitron TI 750 nanoindenter* 1.7AF ™ 2400 DuPont Teflon BioGeneral 72 25.4 25.4 1.27 1.7 AF ™ Extruded2400 Film Solvay Hyflon Cast film 25 25.4 25.4 1.27 0.53 Solexis AD-60 ™DuPont Nafion ™ Literature − 9 7.35 × 10⁻⁴ 2020 film

Each of the polymers listed in Table 2 were prepared as impregnatablesolutions as follows:

TABLE 3 Weight Type Polymer Solvent percent 1 Teflon AF ™ 2400 FC-770 ™3.5 2 Teflon AF ™ 2400 FC-770 ™ 2.5 3 Teflon AF ™ 2400 FC-770 ™ 1 4Hyflon AD-60 ™ FC-770 ™ 2.5 5 Hyflon AD-60 ™ FC-770 ™ 10 6 DupontNafion ™ 2020 Neat 100 7 Dupont Nafion ™ 2020 Isopropanol 50

Various manual methods were used to impregnate the polymer solutions ofTable 3 into the nanotube fabrics of Table 1. The following table setsforth initial observations of the composites.

TABLE 4 Polymer Fabric Solution type (from (from Composite Membrane CaseTable 3) Table 1) Coating Method Drying Method Appearance 1 2 3 72 hoursoak Ambient Stable, strong composite both wet and dry 2 2 4 72 hoursoak followed by Ambient and Stable, strong composite mechanicalimpregnation oven at 60° C. both wet and dry 3 4 3 72 hour soak followedby Ambient and Stable, strong composite mechanical impregnation oven at60° C. both wet and dry 4 5 4 72 hour soak followed by Ambient andStable, strong composite mechanical impregnation oven at 60° C. both wetand dry 5 7 4 72 hour soak followed by Ambient and Stable, strongcomposite mechanical impregnation oven at 60° C. both wet and dry

The following examples illustrate composite membranes of the presentinvention, but are not intended to be limiting as to the materials andmethods of formation for such membranes.

Example 1

The composite membrane of Case 1 from Table 4 was prepared by dipcoating the carbon nanotube fabric in the polymer solution for 72 hourswhile stretched over an aluminum frame. Following submersion for 72hours, the composite material was withdrawn from solution and held in anatmosphere saturated with FC-770 solvent to prevent drying of thecomposite, and the excess solution was allowed to drain from the surfaceof the wet composite. The composite was then allowed to dry underambient conditions under sufficient tension to remove wrinkles.

The dried composite membrane was then laser cut into one inch by twoinch samples for tensile strength measurements, permeability andpermeance testing, and scanning electron microscopy. The following Table5 sets forth testing results of the composite membrane, in comparison toneat polymer films formed from Teflon® AF-2400 through extrusion,solution casting, melt pressing, and laboratory bench casting.

TABLE 5 Permeance (GPU) (1.1 cm dia) Thickness Tensile Permeability,Barrer O2/N2 Type (Microns) (GPa) O2 N2 CO2 O2 N2 CO2 ratio Solution n/a— 1600 780 3900 — — — 2.05 Cast Melt — — 990 490 2800 — — — 2.02 PressedExtruded 38 1.7 585 273 1500 1.6 .75 4.17 2.14 Cast 19 n/a 882 446 208046.6 23.5 109 1.97 Composite 34 5.5 1870 1040 3550 55 31 104 1.79Membrane (TABLE 4, Case 1) Reduced 17 — 934 521 1772 Mathematicallyequivalent to 1.79 Film Data composite membrane

The solution cast and melt pressed data are found in the literature,while the extruded film was sourced from BioGeneral, Inc.

Each of the laser-cut composite membranes was weighed to the nearest 0.1mg, and the thickness determined to the nearest micrometer.

To determine the “reduced film data”, the weight of the carbon nanotubefabric was subtracted from the total composite weight to determine theamount of polymer remaining within the composite structure. The weightof polymer was converted to an equivalent film thickness by densitycorrecting the weight to volume using a factor of 1.67 grams of polymerper cm³. An equivalent film thickness was then determined by dividingthe volume by the length and width of the composite membrane.

The composite exhibits a tensile strength greater than that of the neatpolymer, and dramatically greater than expected under the “Rule ofMixtures”. Applicant believes that an unexplained interaction betweenthe polymer and the carbon nanotubes is responsible for the unexpectedstrengthening property demonstrated in the results table.

A cross-sectional image of the composite is illustrated in FIG. 2, whichshows an asymmetrically formed membrane at section “A” with airinclusions to a depth of approximately four micrometers. Section “B” isapproximately 15 micrometers thick, and is the bulk of the composite,while section “C” is an out of focus inner edge of the cross-section.

To further examine the distribution of the polymer through the bulk ofthe composite, an elemental analysis was performed to account for theconcentration of fluorine, which is only available from the polymer andthe solvent, through little solvent was expected to remain in thecomposite post-drying. The elemental analysis results are set forth inthe following table, wherein “layer 1” is the epoxy potting compoundused to prepare the sample for ion milling:

TABLE 6 Thickness Car- Fluor- Oxy- Observed Material Layer (microns) bonine gen Composition 1 8 80 0 10 Epoxy Potting Compound 2 4 30 70 0Teflon AF 2400 + Fabric 3 15 80 20 0 Fabric + Teflon AF 2400 or FC-770 44 0 0 0 Unfocused inner edge

The composite of this Example 1 exhibits a surprisingly higher tensilestrength than either the base carbon nanotube fabric or the polymer in aneat film format. Moreover, the permeability of the composite membraneis also surprisingly unobstructed by the carbon nanotube fabric,contrary to what would be expected under Nielsen's Model.

Example 2

The composite membrane described in Case 2 of Table 4 was prepared bysoaking the carbon nanotube fabric in solution for 72 hours. Followingsoak, the composite was pressed while in the presence of polymersolution, and subsequently placed on a glass surface. Excess polymersolution was removed from the composite using a squeegee whilecompressing the composite surface to ensure the polymer solution beneaththe composite against the glass was minimized and to remove air bubblesregained between the glass surface and the composite membrane. Thecomposite membrane was then dried at ambient, followed by oven drying at60° C. for four hours.

The following table sets forth strength, permeability, and permeanceresults for the composite membrane:

TABLE 7 Permeance, (GPU) (1.1 cm dia) Thickness Tensile Permeability(Barrer) O2/N2 Type (Microns) (GPa) O2 N2 CO2 O2 N2 CO2 ratio Extruded38 1.7 585 273 1500 1.6 .75 4.17 2.14 Cast 19 n/a 882 446 2080 46.6 23.5109 1.97 Composite 13 5.96 1740 900 4590 134 70 353 1.93 Membrane (TABLE4, Case 2) Reduced Equivalent n/a 940 490 2470 1.93 Film Data to 9microns

The permeability of the composite membrane closely matches the solutioncast film permeability described in Table 5 with reference to a Teflon®AF 2400 film, as that employed in the present Example. In this instance,the weight of polymer remaining in the composite membrane is equivalentof a 9 micrometer neat polymer film thickness.

An ion-milled portion of the composite membrane was further examinedunder electron microscopy. The impregnation technique followed in thisexample closely matches a cast film in O₂/N₂ selectivity, and yet isstronger. An image of the electron microscopy of this composite membraneis shown in FIG. 3. Elemental analysis of the layers illustrated in FIG.3 is set forth in the following table:

TABLE 8 Thickness Car- Fluor- Oxy- Observed Material Layer (microns) bonine gen Composition 1 8 80 0 20 Epoxy Potting Compound 2 <1 30 70 0Teflon AF 2400 + Fabric 3 2 50 50 0 Fabric + Teflon AF 2400 or FC-770 44 0 0 0 epoxy

The elemental analysis shows a distribution of the polymer throughoutthe thickness of the composite membrane, though the polymerconcentration in this Example 2 appears to be higher throughout thethickness of the composite membrane in comparison to the compositemembrane of Example 1. The elevated polymer concentration may beattributed to the mechanical force applied to the composite membrane inthe fabrication technique described in this Example 2.

Example 3

A composite membrane in accordance with Case 3 of Table 4 was preparedusing the procedure of Example 2. The tensile strength of the compositemembrane was compared to a membrane cast from Solution 5 of Table 3:

TABLE 9 Tensile Membrane Strength (GPa) Notes Polymer only 0.99 Purepolymer (Table 3, Type 5) Composite Membrane 6.95 Hyflon AD-60 compositewith (Table 4, Case 3) 10 gram/m² Nanocomp fabric

Example 4

A composite membrane in accordance with Case 4 of Table 4 was preparedusing the procedure set forth in Example 2, and compared to a polymercast film of the solution of Table 3, Type 5. Permeation testing of thecomposite membrane exhibited low selectivity, due to the extremely lowpermeability of the utilized polymer.

TABLE 10 Permeance (GPU) (1.1 cm dia) Permeability, Barrer O2/N2 Type O2N2 CO2 O2 N2 CO2 ratio Solution Cast 51.4 16.5 125 — — —   3:1 (TABLE 3Type 5) Composite 80 60 190 4.9 3.7 11.5 1.3:1 Film (TABLE 4, Case 4)

The data from the “solution cast” neat polymer film was taken fromliterature.

Example 5

A composite membrane in accordance with Case 5 of Table 4 was preparedusing the procedure set forth in Example 2. The composite membrane wastested for strength in both dry and water-saturated conditions, andcompared to films of the neat polymer.

TABLE 11 Tensile Membrane Type Strength (GPa) Notes Nafion 2020 (Table3, Type 6) 7.35*10⁻⁴ Literature Value Nafion 2020 Cast (Table 3, 0.280Cast from 100% Type 6) (dry) solution Composite Film (Table 4, 7.8 DryFilm Case 5) (dry) Composite Film (Table 4, 2.56 Water Saturated Case 5)(wet)

The polymer utilized in this example includes a sulfonatedtetrafluoroethylene that is commonly used in ion exchange membranes topermit ionic species to migrate thereacross. The example compositemembrane may therefore permit transport of cations thereacross,substantially to the exclusion of anions and electrons. Moreover,although the polymer is commonly used in ion conductive membranes, filmsformed from such polymer are also typically highly selective to carbondioxide (CO₂), thereby rendering it a good candidate material for carbonsequestration applications. However, the dry composite membrane is notexpected to demonstrate desired gas permeability or selectivity, becausethe sulfonic acid regions of the sulfonated tetrafluoroethylene polymermust be hydrated so that carbon dioxide can be transported as thedissolved carbonate base, CO₃ ⁻². The sulfonic acid portion of thepolymer in the composite membrane may then carry the ionic speciesacross the barrier.

Example 6

To further enhance ion conductivity of the composite membrane of Example5, a carbon nanotube fabric of Table 1, Case 4 was chemically modifiedin a closed flask with 100 mL reagent grade H₂SO₄ and heated to thesulfuric acid boiling point while covered. The temperature was thenreduced to about 275° C., and the carbon nanotube fabric was allowed toreact in the sulfuric acid for four hours. The mixture was then allowedto cool to room temperature, and was thereafter maintained at roomtemperature for 72 hours. The modified carbon nanotube fabric wasremoved from the sulfuric acid and rinsed with deionized water until therinse water showed a pH>5.

Visual examination of the modified carbon nanotube fabric indicated thatit was easily wetted by water. The modified carbon nanotube fabric wasoven dried at 60° C. for four hours. The modified carbon nanotube fabricwas then impregnated with polymer solution 7 in accordance with theprocedure of Example 2.

The composite membrane was tested for strength in both a wet and drystate:

TABLE 12 Membrane Type Tensile Strength (GPa) Nafion 2020 (Literature)7.35 × 10⁻⁴ Water wet Composite Film 2.6 Dry Composite Film 2.56

The composite membrane was also tested for permeability with wet papertowels placed in the space above and below the wet composite membrane tomaintain humid conditions during permeability testing.

TABLE 13 Permeance, (GPU) Membrane Permeability, (1.1 cm dia) Thickness(Barrer) CO2/N2 Membrane Type Microns N2 CO2 N₂ CO2 ratio Solution Cast38 1 300 — — 300:1 Water wet 11 4.7 224 0.4 20  50:1 Composite MembraneReduced 4 18 850 Film Data

Example 7

A composite membrane was prepared from a 5 g/m² nonwoven carbon nanotubefabric (Case 4, Table 1) and Teflon AF™ 2400 dopant. The carbon nanotubesheet was laser cut into 6 in×6 in squares for mounting over a steelframe. The dopant solution was prepared as 3% by weight Teflon AF™ 2400polymer in Novec™ 7500 solvent from 3M Company of St. Paul, Minn. 3 kgof dopant solution was placed in an immersion tank, which was thenplaced in a water sonication apparatus. In this arrangement, theimmersion tank was placed in a water sonication tank so that ultrasonicenergy emitted by an ultrasonic transducer 314 may be transmitted fromthe water medium in the water sonication tank through the immersion tankinto the dopant solution. The ultrasonic transducer was a Fischer SonicCleaner emitting at 400 W and 30 kHz frequency.

The water sonication tank may be heated to warm the dopant solution to adesired impregnation temperature, typically in the range of 60-90° C.For this application, dopant solution was warmed to 85° C. to achievedesired viscosity levels of the dopant solution.

Once the dopant solution was warmed to the temperature set point,sonication from the ultrasonic transducer was initiated. The framedcarbon nanotube fabric was then immersed in the dopant solution in theimmersion tank for sixty minutes, with a typical immersion time rangingfrom 30-120 minutes. Sonication was then ceased and the framed membranewas removed from the remaining dopant solution in the immersion tank,and allowed to dry at ambient.

A cross-section of an example composite membrane generated throughsonicated impregnation of the dopant solution into a non-woven carbonnanotube fabric is illustrated in FIG. 6, identifying multiple locationsat which spectral analysis was performed to determine relative elementalcomposition among carbon, oxygen, and fluorine. The presence of fluorineand oxygen in the spectral analysis locations within the compositemembrane cross-section indicate the presence of the fluoropolymerdopant. The spectral analysis results are illustrated in FIGS. 7 and 8,illustrating a strong presence of fluorine and oxygen throughout thecross-section of the composite membrane. This evidences the fact thatsubstantially complete dopant penetration through the fabriccross-section was accomplished with the sonication technique. The chartshown at FIG. 7 indicates analysis by Internally normalized DispersiveEnergy X-Ray to show dispersion of the carbon, fluorine, and oxygenthrough the thickness of the composite membrane. Normalizing theconcentration to a value of numeral 1 for each concentration of oxygen,fluorine, and carbon individually removes the bias of their individualresponsive relative to that of carbon. The chart of FIG. 8 indicatesrelative weight percent contribution of each of carbon, fluorine, andoxygen at each spectral analysis location.

Young's Modulus of the composite membranes, as tested by ASTM D882-12,were compared to the nonwoven carbon nanotube fabric alone. The data isillustrated in FIG. 9, evidencing Young's Modulus values for thecomposite membranes of at least about 4 GPa, which is substantiallygreater than Young's Modulus value of the bare Nanocomp 5 g/m² densitynonwoven carbon nanotube fabric, identified as “pure” at 0.5 GPa, or theneat Teflon AF™ 2400 membrane having a thickness substantiallyequivalent to the reduced film thickness of the composite membranesamples, at 1.7 GPa. Accordingly, the composite membranes of the presentinvention far exceed the expected Young's Modulus values of therespective components under the “Rule of Mixtures”.

Example 8

Composite membranes were prepared from nonwoven carbon nanotube fabricand Teflon AF™ 2400 dopant in Novec™ 7500 liquid medium at variousconcentrations and various fabric densities. The carbon nanotube fabricwas laser cut into square samples for mounting over a steel frame. Thedopant composition comprising the dopant and liquid medium was placed inan immersion tank as described in Example 7. Ultrasonic energy wasapplied at various frequencies and powers over various process times toimpregnate the fabric with the dopant composition. The following Table14 sets forth the various materials and conditions used in theultrasonic impregnation:

TABLE 14 Fabric Fabric Dopant Ultrasonic Ultrasonic Process Density SizeConcentration Power Frequency Time (g/m²) (cm²) (wt %) (watts) (kHz)(min) 2 195 2.5 1400 16 120 5 195 2.5 1400 16 120 6 195 2.5 1400 16 1205 195 2.5 4000 20 20 5 195 3 4000 20 20 6 195 3 4000 20 20 2.5 103 2.5400 44 240 2.5 103 3.5 400 44 240 2.5 103 5 400 44 240 5 103 2.5 400 44240 5 103 3.5 400 44 240 5 103 5 400 44 240 10 103 2.5 400 44 240 10 1033.5 400 44 240 10 103 5 400 44 240

The water sonication tank may be heated to warm the dopant compositionto a desired impregnation temperature, typically in the range of 60-90°C., at which the dopant composition exhibits a viscosity suitable topenetrate interstices of the fabric. The experiments of Table 14 wereconducted with the dopant composition warmed to between about 70-75° C.

Once the dopant solution was warmed to the temperature set point,sonication from the ultrasonic transducer was initiated. The framedfabric was then immersed in the dopant composition in the immersion tankfor the designated period of time, ranging from 20-240 minutes.Sonication was then ceased and the framed membrane was removed from theremaining dopant composition in the immersion tank, and allowed to dryto remove the liquid medium from the fabric interstices, wherein thedopant remained incorporated with the fabric to establish a compositemembrane structure. Analysis of the composite structure indicatedsuccessful dopant composition penetration into the interstices of thefabric samples.

Applications

An example application for the composite membranes of the presentinvention is in gas/gas, gas/liquid, and/or liquid/liquid separations.In one embodiment, a system 110 for separating a target gas from aliquid includes a housing 112 defining a chamber 114, and a compositemembrane 116 separating chamber 114 into a permeate side 118 and aretentate side 120. Housing 112 includes an inlet 122 and an outlet 124opening to retentate side 120 of chamber 114. Housing 112 may furtherinclude a gas port 126 opening to permeate side 118 of chamber 114.System 110 may include a pump 128 for evacuating permeate side 118 ofchamber 114 through gas port 126, with pump 128 being fluidly connectedto gas port 126 through, for example, pipe 130.

A liquidous fluid containing the target gas may be delivered toretentate side 120 of chamber 114 through inlet 122 via pipe 132. It isto be understood that the term “pipe”, as used herein, is not intendedto be limiting, and may include any conveyance member of suitable size,configuration, and material to permit the conveyance of fluid to anintended destination.

Composite membrane 116 may be suitably configured to be capable ofseparating the target gas from the liquid at retentate side 120, and maypreferably form a non-porous, permeable barrier exhibiting a permeanceof at least 0.1 gas permeance units (GPU) to the target gas. As is wellknown in the art, separation of the target gas from the liquid atretentate side 120 can be driven by the conditions at permeate side 118of chamber 114. Accordingly, permeate side 118 of chamber 114 may beconditioned to exert a first partial pressure of the target gas that isless than a second partial pressure of the target gas in the liquidousfluid in retentate side 120 of chamber 114. Pursuant to Henry's Law ofpartial pressures and Ficke's law of diffusion, the target gas may bedriven across the barrier defined by the nonporous composite film 116.Pump 128 may be operated to condition permeate side 118 of chamber 114,such as by evacuating permeate side 118 to an extent at which the firstpartial pressure of the target gas is less than the correspondingpartial pressure of the target gas in a liquidous fluid at retentateside 120. In this manner, separated target gas permeated into permeateside 118 may be removed from chamber 114 through gas port 126.

Permeate side 118 of chamber 114 may also or instead be suitablyconditioned by passing a sweep fluid through permeate side 118, whereinthe target gas partial pressure within the sweep fluid is less than thecorresponding target gas partial in the liquidous fluid at retentateside 120, so as to generate a driving force for permeation of the targetgas through composite membrane 116 to permeate side 118. Such a sweepfluid may be introduced to permeate side 118 through a port in housing112.

The above example system is not intended to be limiting as to thestructures and methods of separations contemplated in the presentinvention. Gas/gas, gas/liquid, and liquid/liquid membrane separationsystems are well known in the art, and are considered to be applicableto the systems and methods of the present invention, which uniquelyemploys a composite membrane.

In some embodiments, a composite membrane 216 includes an ionomer thatis capable of transporting ionic species from a first fluid to a secondfluid on opposed sides of the barrier defined by the composite membrane216. A system 210 for transporting ionic species across a barrierbetween a first fluid and a second fluid includes a housing 212 defininga chamber 214, and a composite membrane 216 defining the barrier andseparating chamber 214 into a first side 218 and a second side 220.Composite membrane 216 preferably includes a fabric comprising anon-woven array of intermingled carbon nanotubes and a dopantincorporated with the fabric to form a nonporous permeable composite. Insome embodiments, the dopant is an ionomer that permits transport ofcations across the barrier, substantially to the exclusion of anions andelectrons. Such an ion exchange apparatus may be operated by applying adriving force to chamber 214, wherein the driving force is effective tomotivate ionic species across the barrier defined by composite membrane216. In some embodiments, such driving force may be in the form ofelectrical current generated in chamber 214 between first and secondelectrodes 230, 232 of opposite polarity. As illustrated in FIG. 5,first and second electrodes 230, 232 are positioned at opposed sides ofcomposite membrane 216, with a first electrode 230 disposed in firstside 218 of chamber 214, and second electrode 232 being positioned insecond side 220 of chamber 214. First and second electrodes 230, 232 arecoupled to an electrical energy source 240 for generating a voltagepotential and current passing between first and second electrodes 230,232. An applied current across composite membrane 216 to motivate ionicspecies transport is well understood in the art for membrane-based ionexchange systems.

Other driving forces are contemplated as being useful in motivatingionic species transport across composite membrane 216 between a firstfluid delivered to first side 220 of chamber 214 through inlet 222, anda second fluid delivered to second side 218 of chamber 214 of inlet 224.In one instance, such a driving force may be generated based upon theproperties of the first and second fluids on opposed sides of compositefilm 216. Most typically, however, membrane-based ion exchange systemsemploy an applied electrical current as the force driving ion speciestransport across the membrane.

The invention has been described herein in considerable detail in orderto comply with the patent statutes, and to provide those skilled in theart with the information needed to apply the novel principles and toconstruct and use embodiments of the invention as required. However, itis to be understood that various modifications to the invention may beaccomplished without departing from the scope of the invention itself.

What is claimed is:
 1. A method for preparing a composite gas separationmembrane for separating a gas-liquid mixture, said method comprising:providing a fabric comprising a non-woven array of intermingled carbonnanotubes, wherein said non-woven array defines interstices between saidintermingled carbon nanotubes; providing a dopant in a liquid medium toform a dopant composition; at least partially immersing said fabric insaid dopant composition; and sonicating said dopant composition with anultrasonic transducer such that said dopant composition penetrates saidfabric interstices to an extent sufficient to establish a nonporous butpermeable composite structure with said fabric.
 2. A method as in claim1 wherein said nonporous composite structure exhibits a target gaspermeance of at least 0.1 GPU.
 3. A method as in claim 1 wherein saidfabric has a density of between about 1-20 g/m².
 4. A method as in claim1 wherein said dopant is present in said dopant composition at aconcentration of between about 1-50 weight percent.
 5. A method as inclaim 4, including heating said dopant composition to between 60-90° C.6. A method as in claim 1 wherein said ultrasonic transducer emits sonicenergy at a frequency of between about 10-50 KHz.
 7. A compositegas-separation membrane, comprising: a substrate body having first andsecond generally opposed surfaces defining a thickness therebetween,said body comprising a non-woven fabric of intermingled carbon nanotubesdefining interstices between said intermingled carbon nanotubes; and apolymer dopant distributed within the interstices and throughout saidthickness to form a network that is sufficient to establish a non-porousbut permeable barrier between said first and second surfaces.
 8. Acomposite gas-separation membrane as in claim 7 wherein said polymerdopant is substantially uniformly distributed throughout said substratebody.
 9. A method as in claim 1 wherein said dopant includes anamorphous fluoropolymer.
 10. A method as in claim 9 wherein said liquidmedium is a solvent for said amorphous fluoropolymer.
 11. A method as inclaim 1, including heating said dopant composition prior to sonicatingsaid dopant composition with said ultrasonic transducer.
 12. A method asin claim 11, including heating said dopant composition to a temperatureat which said dopant composition exhibits a viscosity suitable topenetrate said fabric interstices.
 13. A method as in claim 12 whereinsaid temperature is between about 60-90° C.
 14. A method as in claim 12,including removing said liquid medium from said fabric interstices,wherein said dopant remains incorporated with said fabric to establishsaid composite structure.